Editing Westphalian mid-A to mid-C depositional controls, UK Pennine Basin: regional analyses and their relevance to southern North Sea interpretations

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A simple sequence-stratigraphical interpretation could view the mid-A to mid-C Westphalian fill of the Pennine Basin as essentially a single depositional system. Variations in fluvial input and erosion, together with the incidence of coals and marine bands, would be dominantly controlled by marine base level, with progressive regional subsidence generating accommodation space. In this interpretation, even if gross regional thickness variations were influenced by subsidence (see Section 2), the general stratigraphical incidence of the main palaeo-environments would be determined mainly by marine base-level variations.
 
A simple sequence-stratigraphical interpretation could view the mid-A to mid-C Westphalian fill of the Pennine Basin as essentially a single depositional system. Variations in fluvial input and erosion, together with the incidence of coals and marine bands, would be dominantly controlled by marine base level, with progressive regional subsidence generating accommodation space. In this interpretation, even if gross regional thickness variations were influenced by subsidence (see Section 2), the general stratigraphical incidence of the main palaeo-environments would be determined mainly by marine base-level variations.
  
However, the regional analysis outlined above indicates that the fluvial systems, at least, did not conform to this single system of basin fill: their stratigraphical incidence, locations and trends, and thickness and connectivity patterns appear to be normally independent of base-level indicators. This is the case even during periods of relatively lower incidences of sandbodies, such as during intervals of regional peat mantling, when some significant channel systems were still present ([[:File:YGS_CHR_07_WEST_FIG_03.jpg|(Figure 3)]] and [[:File:YGS_CHR_07_WEST_FIG_04.jpg|(Figure 4)]]). There are no regional changes comparable to those reported, for example, by Hampson et al. (1999) for the Namurian C to Westphalian A succession in the Ruhr coalfield, where there is a contemporary progression from fluvial to marine delta-front environments, albeit in a somewhat different setting.
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However, the regional analysis outlined above indicates that the fluvial systems, at least, did not conform to this single system of basin fill: their stratigraphical incidence, locations and trends, and thickness and connectivity patterns appear to be normally independent of base-level indicators. This is the case even during periods of relatively lower incidences of sandbodies, such as during intervals of regional peat mantling, when some significant channel systems were still present (Figs 3, 4). There are no regional changes comparable to those reported, for example, by Hampson et al. (1999) for the Namurian C to Westphalian A succession in the Ruhr coalfield, where there is a contemporary progression from fluvial to marine delta-front environments, albeit in a somewhat different setting.
  
 
Rippon (1996, 1999) considered that the preserved sedimentary volume records the interaction of three distinct depositional regimes:* ''Background lithofacies ''represent a near-continuum through the sedimentary pile, comprising fine-grained clastic rocks and coals, of lacustrine and mire environments. There is a general thinning of all these deposits towards the basin margins.
 
Rippon (1996, 1999) considered that the preserved sedimentary volume records the interaction of three distinct depositional regimes:* ''Background lithofacies ''represent a near-continuum through the sedimentary pile, comprising fine-grained clastic rocks and coals, of lacustrine and mire environments. There is a general thinning of all these deposits towards the basin margins.
 
* ''Channel belt lithofacies ''represent near-continuous inflows into and across the basin, irrespective of the background environment; that is, some channel systems became established during times of mainly lacustrine sedimentation, others during times of mainly mire accumulations. Channel-belt pathways do not primarily reflect basin form, but their thicknesses and connectivities do reflect such a control.
 
* ''Channel belt lithofacies ''represent near-continuous inflows into and across the basin, irrespective of the background environment; that is, some channel systems became established during times of mainly lacustrine sedimentation, others during times of mainly mire accumulations. Channel-belt pathways do not primarily reflect basin form, but their thicknesses and connectivities do reflect such a control.
 
* ''Marine incursions ''occurred infrequently, interrupting both of the above regimes. Faunal variations within the marine strata are considered by the author to be more complex than those implied by Calver (1968), who related progressively more marine faunal populations to basin form (see Rippon 1997). Marine-band thicknesses decrease towards the basin margins, but not to the same degree as the lacustrine mud-stones; this is presumably because the marine sediments were deposited over much less time, so that the effect of reduced subsidence at the basin margins is muted. The directions of marine incursions, which remain uncertain, may not necessarily have related to the (very gentle) gross palaeoslopes as indicated by fluvial-channel pathways. These observations suggest that the marine bands represent distinctive events, controlled by eustatic (global) sea-level rise. Detailed field observations suggest that they were typically not in depositional continuity with the lacustrine deposits. It is considered that the onshore UK coalfields, and the southern North Sea areas, were all significantly distant from a fully marine setting. From this brief review, it is concluded that the preserved basin fill does record the interaction of three essentially different depositional regimes. The relative importance of marine base-level and basin subsidence, with respect to these three regimes will now be assessed.
 
* ''Marine incursions ''occurred infrequently, interrupting both of the above regimes. Faunal variations within the marine strata are considered by the author to be more complex than those implied by Calver (1968), who related progressively more marine faunal populations to basin form (see Rippon 1997). Marine-band thicknesses decrease towards the basin margins, but not to the same degree as the lacustrine mud-stones; this is presumably because the marine sediments were deposited over much less time, so that the effect of reduced subsidence at the basin margins is muted. The directions of marine incursions, which remain uncertain, may not necessarily have related to the (very gentle) gross palaeoslopes as indicated by fluvial-channel pathways. These observations suggest that the marine bands represent distinctive events, controlled by eustatic (global) sea-level rise. Detailed field observations suggest that they were typically not in depositional continuity with the lacustrine deposits. It is considered that the onshore UK coalfields, and the southern North Sea areas, were all significantly distant from a fully marine setting. From this brief review, it is concluded that the preserved basin fill does record the interaction of three essentially different depositional regimes. The relative importance of marine base-level and basin subsidence, with respect to these three regimes will now be assessed.
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=== 3.2 Marine base level ===
 
=== 3.2 Marine base level ===

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